1. Field of the Invention
This invention relates to a process for the production of polyhydric alcohols from 1,3-dioxanes containing at least one hydroxyl group, and more particularly, to a process for the production of polyhydric alcohols containing (n + 2) hydroxyl groups from substituted 1,3-dioxanes containing (n) hydroxyl groups, wherein n is an integer of at least 1.
2. Description of the Prior Art
Heretofore, the most prevalent method of obtaining polyhydric alcohols by the cleavage of the 1,3-dioxane ring has been the solvolysis reaction which is conducted in the presence of a strong acid. Said reaction may be expressed by the following equation: ##STR1##
In the above equation, R represents H or a lower alkyl group, and R.sub.1 to R.sub.8 represent any of H, a lower alkyl group, a cycloalkyl group and an aryl group wherein at least one of R.sub.1 to R.sub.8 is a hydroxyalkyl group.
As is seen from the above equation (1), the solvolysis reaction attains equilibrium and hence the formed carbonyl compound should be eliminated from the reaction system to promote the reaction. Elimination of a carbonyl compound can usually be achieved by conducting the alcolysis reaction in a lower alcohol whereby the formed carbonyl compound is converted to the corresponding low boiling acetal compound which can be easily removed from the reaction system. In this case, however, undesirable side-reactions occur. A considerable amount of ethers are formed through the dehydrolysis reaction between the alcohol as a solvent and the formed polyhydric alcohol and unsaturated alcohols and cyclic ethers are formed through the intramolecular dehydrolysis of the formed polyhydric alcohol in addition to the inevitable dehydrolysis reaction of the alcohol by itself as a solvent. Therefore, the above-mentioned reaction has been found uneconomical for obtaining polyhydric alcohols.
The formation of the by-products as above indicated depends upon the structure of the starting substituted 1,3-dioxane. In general, the more the number of substituents at the 4th and 6th positions on 1,3-dioxane ring is, the more remarkable the occurrence of the side reactions is. In other words, if each of R.sub.3, R.sub.4, R.sub.7 and R.sub.8 in the above formula is a substituent such as an alkyl group, the yield of the intended polyhydric alcohol is extremely lowered. For example, Zhur. Obshchei Khim. 26, 2749-2754 (1956) discloses that methanolysis of 4-methyl-1,3-dioxane in the presence of a sulfuric acid catalyst provides the corresponding glycol in a yield of 82% but methanolysis of 4,4-dimethyland 2,4,4,6-tetramethyl-1,3-dioxane gives the corresponding glycols in yields of only 38.7% and 18.2%, respectively.
When the above reaction is carried out in the presence of water, that is to say hydrolysis, the yield of polyhydric alcohols is very low resulting in no practical application because the reaction rate is low and the formed carbonyl compound is difficult to remove from the reaction system. Where the reaction temperature and/or concentration of catalyst is increased in order to increase the reaction rate, successive side-reactions also occur vigorously so that desirable results cannot be obtained. For instance, in the hydrolysis reaction of 4-hydroxyethyl-4-methyl-1,3-dioxane in the presence of a sulfuric acid catalyst at a temperature of 100.degree. - 110.degree. C., conducted by blowing steam into the reactor to eliminate the formed formaldehyde, dihydropyran is formed as the main product by the successive reactions and is steam distilled and recovered as shown by the following equations. ##STR2##
Thus, according to the prior art, the production of polyhydric alcohols by the cleavage of the 1,3-dioxane ring is economically unsuccessful. Furthermore, the prior art processes which teach the use of organic or inorganic strong acids such as sulfuric and sulfonic acids as a catalyst are also unsuited for industrial operation since equipment corrosion results. In addition, organic carboxylic acids such as acetic acid cannot be used as a catalyst for the production of the corresponding polyhydric alcohols from 1,3-dioxanes, because organic carboxylic acids have weak acidity and hence do not easily promote the cleavage of the dioxane ring.